The present invention relates to a refrigerant cycle apparatus constituted by successively connecting a compressor, a gas cooler, throttling means and an evaporator.
In this type of conventional refrigerant cycle apparatus, a refrigerant cycle (refrigerant circuit) is constituted by successively piping and connecting a rotary compressor (compressor), a gas cooler, throttling means (expansion valve or the like), an evaporator and the like in an annular form. Further, a refrigerant gas is taken into a low-pressure chamber side of a cylinder from an intake port of a rotary compression element of the rotary compressor, and a refrigerant gas at a high temperature and a high pressure is obtained by compression performed by operations of a roller and a vane. This gas is then discharged to the gas cooler from a high-pressure chamber side through a discharge-port and a discharge sound absorbing chamber. The gas cooler releases heat from the refrigerant gas, then this gas is throttled by the throttling means and supplied to the evaporator. The refrigerant is evaporated in the evaporator, and a cooling effect is fulfilled by taking heat from the periphery at this time.
Here, in order to cope with global environment problems in recent years, there has been developed an apparatus which utilizes carbon dioxide (CO2) that is a natural refrigerant even in this type of refrigerant cycle without using conventional fluorocarbon (see, e.g., Japanese Patent Publication No. 7-18602).
On the other hand, when the compressor is stopped after cooling the inside of a chamber in this refrigerant cycle apparatus, a liquid refrigerant is easily accumulated in the evaporator at a lowest temperature in the refrigerant circuit. Especially when the compressor is operated at a constant speed, and is started again in this case, liquid is returned to take the liquid refrigerant pooled in the evaporator into the compressor. There has been a possibility that the compressor compresses liquid and is damaged.
Therefore, in order to prevent the liquid refrigerant from returning into the compressor which results in the liquid compression, an accumulator is disposed between an outlet side of the evaporator and an intake side of the compressor, the liquid refrigerant is stored in this accumulator, and only the gas is taken into the compressor.
In the refrigerant cycle apparatus using carbon dioxide, since the high-pressure side becomes supercritical, the pressure does not become constant at outside air temperature, and rises to about 12 MPa. Especially, when the compressor is operated at a constant speed, the pressure on the high-pressure side further rises at the time of the starting of the compressor (pull-down time), exceeds a designed pressure of the apparatus, and has a possibility that the apparatus is damaged in a worst case. Therefore, a rotation number control (capacity control) of the compressor is executed by an inverter, or an opening of the expansion valve is adjusted, and accordingly a pressure rise on the high-pressure side needs to be reduced to start the compressor.
On the other hand, when an inexpensive capillary tube is used in the throttling means, in addition to the above-described abnormal rise of the pressure on the high-pressure side, there has been a problem of an increase of power consumption, because the rotation number of the compressor needs to be raised in order to lower an evaporation temperature of the refrigerant in the evaporator.
Moreover, when the refrigerant cycle apparatus is used as a cooling apparatus for cooling a refrigerator or a vending machine, a compression ratio of a carbon dioxide refrigerant becomes very high, and the temperature of the compressor or that of a refrigerant gas discharged in a refrigerant cycle becomes high. In this relation, it has been difficult to obtain a desired cooling capability (refrigerating capability) in the evaporator.
Furthermore, in the refrigerant cycle apparatus using carbon dioxide described above, since the high-pressure side becomes supercritical, the pressure on the high-pressure side rises regardless of the outside air temperature, and exceeds the designed pressure of the apparatus, and the apparatus is possibly damaged in the worst case. Therefore, the rotation number of the compressor is controlled, or a flow path resistance of the throttling means is adjusted, and accordingly the pressure on the high-pressure side has been controlled so as not to exceed the designed pressure of the apparatus.
On the other hand, when the accumulator is disposed on a low-pressure side of the refrigerant cycle, a larger filling amount of refrigerant is required. A problem has also occurred that an installation space is enlarged. Therefore, the rotation number control (capacity control) of the compressor is executed by the inverter, or the opening of the expansion valve is adjusted, and an amount of refrigerant taken into the compressor at the starting time is reduced in starting the compressor. Accordingly, an inconvenience that the liquid refrigerant is sucked into the compressor needs to be prevented.